The present invention relates to the field of metal plating, and more specifically, to a multi-layer plating and method for improving the corrosion resistance of articles having a ferrous metal substrate and which exhibit satisfactory appearance for commercial utilization, preferably either bright or dull chromium or chromium-substitute finished articles.
A persistant problem in the technical field has been to provide ferrous metal articles which are resistant to corrosion which inherently attacks such articles in normal usage. With some ferrous metal articles, the need to provide the maximum degree of corrosion resistance is particularly great in view of the corrosive environments in which they are utilized. For example, metal fasteners which are utilized extensively in automotive as well as other industrial applications are typically exposed to corrosive salts and other corrosive agents which would result in rapid deterioration, both functionally and aesthetically, of such articles.
It is known that the corrosion resistance of ferrous metal articles, particularly steel fasteners, can be obtained by applying metallic coatings, either in single or multiple layers, over the ferrous metal substrate. Such a layer or layers of metal provide either greater inherent resistance to corrosion than the ferrous metal substrate, or they are "sacrificial" in that they are preferentially attacked by corrosive agents.
For example, it is known that improvement in the corrosion resistance of a ferrous metal substrate can be achieved by applying sequential layers of copper, cadmium, copper, nickel, and chromium or a chromium substitute comprised of alloys of tin-nickel, tin-cobalt or tin-cobalt-nickel. This plating sequence has been utilized in the U.S. automotive industry, particularly on steel fasteners.
However, further improvement in the corrosion resistance even of such plated ferrous metal articles is still necessary, particularly in view of the present trend of automobile owners to use their vehicles over a longer period of time and greater mileage. Also, improvement is needed in the reliability with which such multi-layer platings can be applied to ferrous metal articles. It is particularly difficult to achieve good, reliable corrosion resistance by multi-layer platings on ferrous metal articles which have a rough, uneven surface, such as results from producing the article by heading operations. The surface of these types of articles can contain pits, laps, cracks, scratches, surface defects and other irregularities which make uniform plating onto the surface difficult. Conventional plating sequences are incapable of completely and uniformly coating the aforementioned defects.